The Stanford Encyclopedia of Philosophy has updated their entry on Modal Logic, HERE. Also, for those interested in modal logic and its application to questions within the philosophy of religion, one might find this book, Logic and Theism, interesting (HERE).
My interest in logic and its application to metaphysical theology stems from my early readings of Hegel, actually. As time went on I discovered Hartshorne and learning modal logic became invaluable. I think what cinched things for me - that one had to have a good grasp of logic in order to do good metaphysics (let alone theology) - was when I took two back to back seminars in Medieval philosophy during graduate school.
Since then I've reflected on logic and its application to metaphysics and religion frequently (for example, see THIS post).
Here is one of my favorite Hartshorne quotes:
"Logic is the backbone of philosophy. And nothing is quite clear
logically unless it can be put mathematically. Ideally at least, a
philosopher should be a mathematician and logician as well as
metaphysician. Perhaps this could be said of Plato, certainly of
Leibniz, Peirce, and Whitehead."
- Charles Hartshorne, Creative Synthesis & Philosophic Method